It's expected that sender can only specify field/signature name and won't be able to customize field/signature ID when leveraging Position Extraction feature. Thus, if a subsequence workflow requires to reference the field/signature ID (e.g. when adding field conditions), you will need to pull the package JSON, retrieve the mappings between field names and IDs, then update the transaction accordingly.
This sample code showcases you the necessary steps in order to achieve this. If you encountered any problem using the code or want to request a sample code in .NET sdk, or RESTful code, please comment below.